Filters typically include porous filter media which is permeable to a fluid medium (liquid or gas) but is impermeable to particulate matter. The media is usually relatively thin paper (of cellulosic or synthetic material, e.g., polyester).
The removal of particulate matter, e.g., dust, is accomplished by passing the fluid, e.g., air, through the filter media. Dust collects on the filter media thus gradually filling its pores and increasing the restriction of the filter, thereby increasing the pressure drop across the filter and the load on the air-moving fan or blower, or reducing air flow through the filter.
Thus, a successful filter must have a large enough area of filter media to keep the restriction to an acceptably low level for an acceptable time of use; must be very efficient in terms of capturing particulate matter; and must be capable of either being replaced or of being cleaned at sufficiently frequent intervals to prevent dirt from accumulating to a point where the filter is too restrictive.
To meet the objectives outlined above, pleated filters were developed. Pleated filters typically include cellulosic or synthetic filter media which is quite thin and is folded in zigzag (accordian-like) fashion to produce a plurality of pleats. Each pleat is made up of a pair of rectangular panels, with fold lines separating the panels, and the pleats and fold lines of a pleated filter usually run vertically (substantially perpendicular to the end caps, discussed below). To provide even more area, while also holding the panels apart to insure maximum access, corrugated media was introduced. Clearly, pleated filters, whether corrugated or not, can have very large surface areas compared to their volumes and thus can provide acceptably low flow restrictions.
A typical pleated filter cartridge or element is made up of a pair of metal or plastic end caps spanned by pleated filter media, with the pleats and fold lines normally running from one end cap to the other, with the corrugations at right angles to the pleat tips (or parallel to the end caps) to provide for open air flow to the entire panel width. The ends of the media are potted or sealed into the end caps using a potting material such as plastisol, urethane, hot melt or epoxy. The end caps can either be circular, obround or elliptical, in the case of a cylindrical filter element, or rectangular, in the case of a "panel" filter. See, e.g., U.S. Pat. No. 4,204,846, where rectangular panel filters are employed in a cabinet cleaner. The filter media edges are sealed or potted in end caps to facilitate mounting and sealing replaceable filter elements in an air duct such that dirty air has to flow through the filter media and cannot by-pass it.
Although pleated filters have substantially met the objectives outlined above, as a class they suffer from some drawbacks. For example, pleats tend to collapse as a result of the differential pressure they must withstand, resulting in less effective surface area available for filtering. Pleat collapse problems have been addressed in various ways. One successful technique, disclosed in U.S. Pat. No. 4,443,235, involves the use of hot-melt deposits or spacers applied to the pleat panels to maintain a minimum separation therebetween.
Another problem associated with pleated filters as a class is that they, like all filters, tend to become plugged or clogged during use. In the event of substantial plugging, a filter element can be replaced or cleaned. Fortunately, there are several suitable techniques for cleaning plugged filter elements. For example, U.S. Pat. No. 4,331,459 discloses a method of pulsing air backwards through a pleated filter to dislodge the dust cake on the dirty side of the media.
Thus, several problems associated with pleated filters have been successfully addressed, at least to a degree. One problem, however, has defied solution for many years: some of the pleated filter elements of a given production run tend to leak. More specifically, pleated filters sometimes allow dirty air to shunt around their media, thereby allowing dirty air to pass through. It has long been recognized that the leak problem is associated with potting the ends of the pleats during filter element construction.
Occasionally, and intermittently, voids have occurred in the potting material such that dirty air can flow through the voids in the potting material rather than through the filter media. Since the air that flows through the voids or "short circuit paths" is not filtered, this reduces the overall filtering efficiency of the cartridge or element, particularly in view of the fact that air, like all fluids, will choose the path of least resistance.
While the leak problem associated with pleated filters occurs only intermittently, the problem seems to occur most often when corrugated pleated media is used. As discussed above, corrugated filter media typically includes corrugations which run horizontally, perpendicular to the fold lines and the pleats. The corrugations serve to separate the pleats and the panels thereof to ensure that all of the filter media comes into play (see FIG. 2). The leak problem also seems to occur more often with high pleat densities, although even then only some elements out of a set of apparently identical elements in a production run will exhibit the problem. This explains why the problem has resisted solution for so long.
Several solutions to the pleat potting problem have been proposed. For example, it was once thought that overheating the adhesive-plastisol potting formulation at the metal end cap/plastisol interface during the plastisol curing process was responsible for the voids and leaks. While overheating can cause gas generation, the effect is somewhat different and in any event lowering the hot plate temperature to the point where adhesion was poor to lacking, and the plastisol inadequately cured, failed to eliminate voids and leaks.
The leak problem associated with pleated filters was also blamed on the potting material itself. Several different materials were tried, but the problem intermittently reoccurred.
These and other proposed solutions have been tried, to no avail. The leak problem has resulted in the scrapping of finished filter elements, on occasion. Even minor filter media leaks are particularly unacceptable to the military as they, not surprisingly, have very strict specifications as to filter efficiency and reliability.
In response to this long-standing problem, the applicant examined several filters by removing the end caps and cutting out sections of the potted filter media. This examination revealed voids in the potting material and a pattern of non-uniform penetration of the spaces between the pleats by potting material, with a noticeable absence of potting material adjacent the voids. Upon observation of these phenomena, the applicant theorized that the edges of some of the adjacent corrugated panels were acting together to form "check valves" capable of limiting or altogether blocking the flow of a potting material. FIGS. 4A and 4B, "stop action" views of a prior art filter during assembly, illustrate the applicant's theory diagrammatically. In FIG. 4A, the lower vertical arrows show the relative motion of the plastisol as the media assembly is inserted into the plastisol. As the media enters the plastisol, the surface of each pleat panel is deflected as shown by the small horizontal arrows (just as a rudder is deflected by the flow of water against it). This tends to pinch off the flow of plastisol at "linear check valves" 28 where the edges are pushed together if the media edge happens to be close enough to the maximum amplitude of the corrugation wave pattern. At the same time, the deflection of the media that closes off flow at "check valves" 28 opens up flow at 29 even more. If, after this occurs, the relative motion is reversed, atmospheric air tries to fill in the space vacated by the media/wet plastisol unit. The path of least resistance is through the spaces 28 between the pleats where the plastisol did not penetrate. Reverse motion might occur through tilting the media pack or correcting a hung-up liner, through media shrinkage or even through clamp relaxation. When the assembly is pushed back in (see FIG. 4B), the initial situation is restored and the air cannot readily retreat the way it came in. This results in the large voids found in bad elements. Many times (maybe even most of the time) these voids do no real harm, but on occasion an opening to the opposite side of the filter occurs, whether through passage of air from one of these bubbles through a pleat space previously filled (or only partly filled) with plastisol, or by some other fault.
Once the "check valve" mechanism was recognized, it was apparent that the voids and leaks could be avoided by eliminating all forms of repositioning or withdrawal of the media assembly from the potting material. This follows from the idea that withdrawal of the media from the potting media required the vacated space to be filled by something else, and that where pleat penetration had not occurred the most likely replacement would be air flowing under atmospheric pressure between those open pleats (see FIG. 4B). However, despite improved fixtures and operator awareness, this was not 100% achievable. It would also leave alternating high and low (or no) pleat penetrations, a weaker construction.
The present invention addresses the problems associated with potting pleated filters. The invention is particularly aimed at potting corrugated pleated filters media in such a way as to eliminate voids in and leaks through the potting material.